Warming in Deep Southern Ocean Linked to Sea-Level Rise

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Warming waters in the deepest parts of the ocean surrounding
Antarctica have contributed to sea-level rise over the past two
decades, scientists report today (Sept. 20).

In an attempt to pinpoint all culprits for the rising oceans,
scientists analzyed warming trends in the abyssal ocean - below
about 3,300 feet (1,000 meters), said study team member and
oceanographer Sarah Purkey of the University of Washington in
Seattle.

The scientists found that the strongest deep warming occurred in
the water around Antarctica, and the warming lessens as it
spreads around the globe. The temperature increases are small -
about 0.05 degrees Fahrenheit (0.03 degrees Celsius) per decade
in the deep Southern Ocean, and less elsewhere. But the
large volume of the ocean over which they are found and the
high capacity of water to absorb heat means that this warming
accounts for a huge amount of energy storage.

If this deep ocean heating were going into
the atmosphere instead - a physical impossibility - it would
be warming at a rate of just over 5 degrees F (3 degrees C) per
decade.

This amount of energy would be the equivalent of giving every
person on Earth five 1,400-Watt hair dryers, and running them
constantly during the 20-year study period, said study team
member and oceanographer Gregory Johnson of the National Oceanic
and Atmospheric Administration (NOAA).

Sea level has been rising at around one-eighth of an inch (3
millimeters) per year on average since 1993, with about half of
that caused by the ocean expanding as it's heated, and the other
half due to additional water added to the ocean, mostly from
melting continental ice.

The oceanographers note that deep warming of the Southern Ocean
accounts for about one-twentieth of an inch (1.2 mm) per year of
the sea-level rise around Antarctica in the past two decades.

The authors note that there are several possible causes for this
deep warming: a shift in Southern Ocean winds; a change in the
density of what is called Antarctic Bottom Water (which would
change how much gets mixed with surface waters); or how quickly
that bottom water is formed near the Antarctic, where it sinks to
fill the deepest, coldest portions of the ocean around much of
the globe.

The study draws on temperature trends between the 1990s and 2000s
in the deep
Southern Ocean. Though there are no continental boundaries
there, and all oceans contribute water to the Southern Ocean, its
distinct circulation makes the area a separate water body.
This study shows that the deep ocean is taking up about 16
percent of the energy that the upper ocean is absorbing.

To study how much heat was building up in the Southern Oceans,
the researchers divided the ocean into 28 sections, or basins,
for which they computed warming rates. As a part of a large
international research mission, researchers sailed across the
ocean, stopping every 30 miles (48 kilometers) to lower their
instruments into the deep ocean to take measurements for four
hours.

"It was like crossing the ocean at a jogging pace," Johnson said.

The three southernmost basins showed a strong statistically
significant abyssal warming trend, with that warming signal
weakening to the north in the central Pacific, western Atlantic
and eastern Indian Oceans, the researchers said. Eastern Atlantic
and western Indian Ocean basins showed slight cooling trends,
though the results weren't statistically significant, meaning
they could be due to measurement or other errors.

The study is detailed in a recent online edition of the Journal
of Climate.